Force fields

Force fields consist of (hopefully) transferable parameters for molecular sub-units, usually at the atomistic level. They are designed to be applicable to a variety of molecular systems, in particular for flexible molecules, over a (limited) range of thermodynamic conditions. Force fields usually consist of two parts; a functional form, where aspects of the molecular geometry, such as bonds, angles, torsions etc, are each assigned a mathematical function. The second aspect is a set of parameters, whose values vary depending on the atomic elements in question. It is not uncommon to exclude hydrogen atoms using the united-atom approximation.
Sometimes, for very large systems consisting of many large molecules, coarse graining is used. Force fields are often used to study macromolecules such as polymers and proteins.